Skype

I use Skype a lot, Skype to Skype is free and fees are low to call landlines and mobiles, the voice quality is very good also.

With Skype you can make free calls over the internet to other people on Skype for as long as you like, to wherever you like. It is free to download.
Skype allows users to make telephone calls over the Internet to other Skype users free of charge and to landlines and cell phones for a fee. Additional features include instant messaging, file transfer, short message service, video conferencing and its ability to circumvent firewalls.

www.skype.com